More precisely, the present invention relates to a globe valve which is usable, in particular, in nuclear installations and which is easy to disassemble, and in particular its seat is easy to disassemble.
In nuclear reactors of the pressurized water type, the energy produced by the fission reaction of the nuclear material in the core of the reactor is transmitted to a primary heat exchanger by a primary flow of water between the reactor vessel and the primary heat exchanger. Although the fuel material is enclosed in sheathing for preventing loss fuel material, there is always the possibility of cracks appearing in said sheathing, thereby allowing fission products to escape into the primary water. It is thus necessary for all the parts of the primary circuit to provide excellent confinement.
Further, it is known that in this type of nuclear installation, the primary circuit assembly is enclosed in a confinement chamber having a certain amount of radiation propagating therein. Human intervention inside the confinement chamber must thus be unusual and must take as little time as possible, in other words any act that needs to be performed on any of the components of the primary circuit must be easy to perform. Furthermore, it is important to have advance knowledge of the approximate duration of any such act.
Finally, because of the radiation which exits inside the primary reaction vessel, and in particular because of neutron radiation, it is extremely desirable for the primary water to contain no particles of material likely to give rise to radioactive products. It is thus necessary to ensure that none of the components of the primary circuit is made of a material likely to give rise to radioactive products under the effect of such radiation. This applies, in particular, to cobalt.
In such applications, current implementations of globe valves having compressed packing generally comprise a housing of embossed stainless steel constituted by a body and a yoke cap which are screwed and welded together, with the weld providing static external sealing.
In current embodiments, with the seat metallurgically deposited on the body, it is not possible to change the shape and the nature of the sealing bearing-surface since the apparatus is welded to its pipe-work in order to fit it better to local working conditions, for example.
It should also be observed in current implementations that lapping the seat of a valve which is already in place in its circuit requires a long and difficult prior operation for displacing the valve head. Special tools are required for grinding away the weld on the sealing lips and for unscrewing the yoke-cap. Finally, this type of action can generally be done no more than three times, because the lips are so small.
Further, it should be observed that reconditioning the packing is generally long and difficult, since it requires an extraction tool and since the replacement sealing rings must necessarily be split, which considerably reduces their effectiveness.
Three principal criteria can be selected from the various criteria that need to be satisfied by valves mounted in various parts of the primary circuit of such a nuclear reactor. It is necessary for the seat of the valve or for its packing to be rapidly changeable by means of simple operations. It is necessary that the valve should not constitute a weak point in maintaining the confinement of the primary circuit, in other words the valve must provide excellent sealing, in particular relative to the outside. Finally, it is necessary to avoid using materials in the valve assembly which are liable to produce radioactive products under the effects of radiation.
Given these requirements, a main object of the present invention is to provide a globe valve from which it is easy to extract the seat and packing while still maintaining good sealing, and to provide a valve which avoids using any cobalt.